MINNESOTA DEPARTMENT OF NATURAL RESOURCES

Technical Appendix to Part B

REGIONAL HYDROGEOLOGIC ASSESSMENTOTTER TAIL AREA, WEST-CENTRAL MINNESOTA

REGIONAL HYDROGEOLOGIC ASSESSMENT SERIES RHA�5

PART A(Published separately by the Minnesota Geological Survey)

Plate 1, Surficial geologyPlate 2, Quaternary stratigraphy

PART BPlate 3, Surficial hydrogeology

Plate 4, Geologic sensitivity to pollution of near-surface ground waterTechnical Appendix, Water chemistry and residence time

St. Paul2002

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Table of Contents

WATER CHEMISTRY AND RESIDENCE TIME ....................................................................... 1INTRODUCTION ................................................................................................................ 1REGIONAL WATER MOVEMENT .................................................................................... 1WATER TYPE AND QUALITY ......................................................................................... 3LAKES AND INTERACTION WITH GROUND WATER ............................................... 4GROUND-WATER RESIDENCE TIME ............................................................................. 6SENSITIVITY MAP PREPARATION .............................................................................. 10

GLOSSARY .................................................................................................................................. 13

List of Tables and Figures

FIGURE 1. Sites with noted ground-water characteristics ............................................................. 4FIGURE 2. Lake and well sites with stable isotope data ................................................................ 5FIGURE 3. Stable isotope data for the study area .......................................................................... 5TABLE 1. Stable isotope data for selected samples ....................................................................... 5FIGURE 4. Comparison of barium and strontium by tritium age and hydrogeologic setting ........... 8FIGURE 5. Chloride and sulfate in Minnesota waters .................................................................... 9FIGURE 6. Recharge environment ................................................................................................. 9

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GLOSSARYage dating � Expressing the age of water in years by measuring radioactive elements (e.g., tritium and carbon-

14) and their decay products.aquifer � Geologic material capable of yielding a usable quantity of water to a well.buried aquifer � A confined or unconfined aquifer covered by at least 10 feet of low-permeability sediments in a

Quaternary setting.carbon-14 � A radioactive isotope of carbon (14C) with a half-life of 5,730 years; used for age-dating.carbonate rock � Rock containing the carbonate anion and consisting chiefly of carbonate minerals such as

calcite and dolomite.contamination � Degradation of natural water quality as a result of human activity.discharge � The volume of water flowing through a given cross section in a given time period.drinking water standards � National Primary Drinking Water Standards issued by the Environmental Protec-

tion Agency for public water systems. The standards establish maximum contaminant levels for severalinorganic and organic substances that could pose a health risk. Secondary standards list maximum con-taminant levels for substances that affect taste, odor, and staining. For current standards, seewww.epa.gov/safewater/standards.html.

Eh � Redox potential using a hydrogen scale. The �E� means electromotive force and �h� means hydrogen scale.electrical conductivity � The ability of a substance to conduct an electrical charge.fractures in till � Secondary porosity in till produced by weathering.hardness � A measurement of the property of water that causes formation of insoluble residue when the water is

used with soap. It is primarily the sum of concentrations of calcium and magnesium ions.hydraulic conductivity � A measure of the ease with which a fluid flows through porous geologic material.kettle lake � A lake in a depression in glacial drift formed by the melting of an isolated ice block.map polygon � A cartographic feature used to represent an area. A polygon has attributes that describe it.model � A system of postulates, data, or inferences presented as a simulation or representation.parameter � A physical property or set of physical properties whose values determine the characteristics of

something.permeability � A measure of the ability of a geologic unit to transmit fluids.pollution sensitivity � The general potential for ground water to be contaminated.recharge � 1. The entry of water into the saturated zone at the water-table surface, together with the associated

flow away from the water table within the saturated zone. 2. The entry of water into an aquifer.recharge (ground water) area � Area at or near the land surface through which water moves to replenish an

aquifer or other ground-water resource.residence time � 1. The time water has been underground; the time elapsed from when water left the atmo-

sphere until it is discharged from the ground-water system. 2. The time water resides in a lake.saturated zone � The portion of the subsurface in which all voids and cracks in geologic materials are com-

pletely filled with water.stable isotopes � Isotope of an element that does not decay. Isotopes of oxygen (16O and 18O) and hydrogen

(1H and 2H) are examples of stable isotopes used to understand the sources of water or the processes thathave affected the water.

static water level � The level to which water will rise in an unpumped well that is open to a single aquifer.surficial aquifer � A water-table aquifer in sediment that is exposed at the land surface.travel time � The time required for water or a contaminant to move from a source at the land surface to an

aquifer or other target.unsaturated � Voids and cracks in geologic materials are not completely filled with water.water-table aquifer � The uppermost aquifer that has a water table; more generally, an unconfined aquifer.

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WATER CHEMISTRY AND RESIDENCE TIMEbyJulie Ekman, DNR WatersandScott Alexander, University of Minnesota

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INTRODUCTIONThis appendix describes regional water move-

ment, water types and quality, lakes and interactionwith ground water, ground-water residence time, andsensitivity map preparation. Data sources includedwater-level measurements and chemistry analyses ofwater samples from wells and lakes in the Otter Tailstudy area. A glossary at the end of the appendix de-fines terms used on the plates and in the appendix.The study area is bounded by 46o to 47o latitude and95o to 96o longitude in west-central Minnesota andincludes most of Becker and Otter Tail counties andparts of Douglas, Grant, Todd, Wadena, and Hubbardcounties. The plates and appendix provide informationthat is useful for planning and environmental protec-tion efforts.

Glacial deposits provide the framework for thehydrogeology of this study area. Sediment textureinformation from Plate 1 of Part A and information ontopography were integrated into four hydrogeologicsettings: hummocky moraine (map units cs, cd, cm,os, od, om, lgs, lgt, lgd, lgm, ugs, ugd, and ugm),ground moraine (map units cg and cw), collapsed out-wash (map units oo, op, lgo, lgp, ugo, and ugp), andoutwash plain (map units cc, oow, oc, lgow, lgc, ugc,and ha).

Glacial melting and the subsequent collapse ofsediments produced the hummocky moraine settingcharacterized by high topographic relief and unsortedmaterials. Typical sediment textures in this settinginclude sandy loam, loam, and loam to clayey loam.Water scoured or overriding ice eroded the underlyingtills of the ground moraine forming drumlins in muchof this setting. The rolling, low-relief topography in theground moraine setting consists of sandy loam sedi-ments. Surficial sands and gravels deposited by broadmeltwater streams characterize the collapsed out-wash and outwash plain settings. Subsequent meltingof buried ice blocks formed the depressions, manynow occupied by lakes, which are typical of the col-lapsed outwash setting.

REGIONAL WATER MOVEMENTPermeability and topography of sediments influ-

ence ground-water movement. Water-level infor-mation and chemistry analyses were compared withhydrogeologic setting and well depths to evaluate thisinfluence.Regional Drainage

The lower topographic relief in the outwashplain and ground moraine settings in the eastern partof the study area contributes to generally simpler flowsystems than in the collapsed outwash and hummockymoraine settings to the west and south. The groundmoraine setting has extensive drumlins and lower per-meability sediments that channel water across thesurface through numerous streams to the Leaf andRedeye rivers. These major rivers, along with theLong Prairie and Straight rivers, provide dischargeoutlets for ground water in the outwash plain andground moraine settings. Ultimately, these watersmove east, to the Mississippi River. Ground water inall four settings in the extreme southeast moves to theEagle and Long Prairie rivers and ultimately to theMississippi River system.

Ground-water movement in the water-table sys-tem in the collapsed outwash and hummocky morainesettings of the western and southern parts of thestudy area is dominated by local flow systems con-nected with lakes and wetlands. Many of these lakesoccur in closed depressions in the hummocky mo-raine. A few lakes connect with the Otter Tail orPomme de Terre rivers that ultimately move water tothe Red River of the North and Minnesota River, re-spectively. In the collapsed outwash setting, manylakes are connected by the Otter Tail or Buffalo riv-ers or their tributaries.Influence of Hydrogeologic Setting

Ground-water movement is relatively rapidthrough the permeable surficial sand and gravel sedi-ments of the outwash plain. In this setting, all of the

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1 Total dissolved solids (TDS) is the sum of dissolved ions in parts per million (ppm). The bicarbonate ions are convertedto carbonate by a gravimetric factor of 0.4917 to approximate the residue on evaporation method. This computed value ofcarbonate is used in the summation of dissolved ions (Hem, 1992).

Radioactive and Stable IsotopesIsotopes of a particular element have the same number of protons but different numbers of neutrons. Isotopes

are called stable if they are not involved in any natural radioactive decay. Stable isotopes are used to understand watersources or the processes that have affected them. Radioactive isotopes decay at known rates and can be used to mea-sure the length of time water has been removed from the atmosphere.

The important stable isotopes are oxygen (16O and 18O) and hydrogen (1H and 2H). The mass differences be-tween 16O and 18O or 1H and 2H can cause the concentrations of these isotopes to change (fractionate) during evapora-tion and precipitation, resulting in different 16O/18O and 1H/2H ratios in rain, snow, rivers, and lakes. The d2H and d18Ovalues (d denotes the relative difference from standard mean ocean water) in precipitation generally plot close to astraight line known as the meteoric water line. The departure of 18O and 2H values from the meteoric water line canindicate evaporation or mixing of water from different sources.

Tritium (3H) is a radioactive isotope of hydrogen that is produced naturally by cosmic radiation in the upperatmosphere. Before 1953, naturally occurring levels were around 3 to 5 TU (1 TU = 1 tritium atom per 10 18 hydrogenatoms). Once created, tritium decays with a half-life of 12.43 years. Atmospheric nuclear weapons testing between 1953and 1963 released excess tritium into the atmosphere. Atmospheric tritium concentrations peaked between 1962 and1965. Ground water in Minnesota has been classified into three age categories based on tritium levels (Alexander andAlexander, 1989). Waters with tritium levels of 10 TU or higher are �recent waters� that entered the ground water since1953. Waters that now contain less than 0.8 TU are classified as �vintage waters� and entered the ground water priorto 1953. Waters with 0.8 to 10 TU are �mixed waters� of the previous two. Recent water that recharged during the1950�s and 1960�s may contain significantly more than 10 TU. These waters containing 20 to 100 TU are termed �ColdWar era�. The advent of modern agriculture, with increasing use of fertilizers and pesticides, also starts in the1950�s,and 3H is useful for identifying water resources that are vulnerable to contamination from these farming practices.

Carbon-14 (14C) is a radioactive isotope of carbon with a half-life of 5,730 years. It can be used to establishground-water residence times as old as 35,000 years. It is produced in the earth�s upper atmosphere and enters theground-water system with precipitation. The amount of 14C in the atmosphere remains relatively constant over time;therefore, that amount can be compared with 14C in a ground-water sample to estimate how long the water has beenisolated from the atmosphere. Unlike dating a piece of charcoal, the carbon dissolved in water is subject to alterationby physical and chemical processes. Corrections can be made for inorganic sources (carbonate rock), calcite precipita-tion, and methane production. Physical mixing along flow paths also reduces precision of dates.

water samples from the six wells that are less than100 feet deep (approximately the maximum thicknessof the sand and gravel sediments) had detectable tri-tium (greater than 0.8 tritium units [TU]) indicatingthat some or all of the water within the outwash plaindeposits has infiltrated during the past 50 years. (Forinformation on tritium and ground-water age, see Ra-dioactive and Stable Isotopes, above.) Four of thesesamples had recent water (less than 50 years old);the two with mixed water are close to lakes wheredeeper, older ground water moves upward. Threeground-water samples with vintage water (more than50 years old) were from deeper wells completed insand and gravel deposits within the till below the surf-icial outwash deposits. The water in these three vin-tage-age samples also had higher concentrations oftotal dissolved solids1  (TDS), ranging from 325 milli-grams per liter (mg/L) to 595 mg/L TDS, comparedwith samples from the six shallower wells (190 mg/Lto 310 mg/L TDS).

Water samples from wells close to each other inthe less permeable sediments of the ground morainesetting showed a decrease in tritium concentrationsand an increase in TDS with depth. Vintage waterdominated in samples from depths greater than 135feet. Overall, ground water moves relatively slowly inthe ground moraine sediments; however, fractures inthe till, root zones, and animal burrows can createpreferential flow paths for ground water. These het-erogeneities result in a secondary porosity that per-mits water and contaminants to reach the water tablerapidly, bypassing the clay-rich glacial matrix.

The moderate relief of the collapsed outwashsetting and its permeable sand and gravel sedimentspromote vertical ground-water movement. Measur-able tritium was found in all but one ground-watersample from wells to 130 feet deep. Near the north-central boundary of the study area, in a relatively low-relief area of collapsed outwash, a sample from a

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shallow well (64 feet deep) had vintage water, indi-cating that there can be pockets of protected water incoarse sediments possibly due to slow, lateral ground-water flow. In the collapsed outwash setting, manylakes are connected by streams and interact withground water. Ground water can discharge to a lake,or lake water can recharge ground water. This inter-action can vary by position on a lake and by seasonalvariations in precipitation (see Lakes and Interactionwith Ground Water, below).

Short ground-water flow paths connected withsurface water typify the shallow ground-water flow inthe hummocky moraine setting. High relief, loam toclay loam sediment, and heterogeneities such as bur-ied sand and gravel deposits produce these short flowpaths that are not well integrated into the regionalground-water flow system. The short flow paths areindicated by the highly variable water elevations oflakes within small geographic areas. Because theshallow ground-water flow directions in the hum-mocky moraine represent localized conditions, gener-alizations about water age and changes of waterquality with depth could not be made because watersamples from wells were likely from different localflow systems. Collapsed sediment from glacial melt-ing can create interconnected buried sand and graveldeposits. An example of this appears on Plate 3,cross-section C�C�, from Clitheral west. Water thatenters at the surface can be channeled through thesehigh-permeability flow paths allowing movement ofrecent water and contaminants to greater depths andto greater lateral distances than would occur in ahomogeneous till (see condition 3, Figure 5, onPlate 4).

WATER TYPE AND QUALITYThis section summarizes the water types and

water quality in the Otter Tail study area. The sum-mary is based on 88 water samples that were ana-lyzed for major ions. A compilation of the waterchemistry data from this study is available from DNRWaters.Major Ions

Analysis of the major ions indicates that thewaters sampled in this study area resemble the natu-ral waters in most of Minnesota. Calcium (Ca), mag-nesium (Mg), and bicarbonate (HCO3) were thepredominant ions in most samples of ground water

and all samples of lake water. As precipitation infil-trates through the soil zone, the water interacts withdissolved carbon dioxide gas resulting in relativelyhigh concentrations of bicarbonate. Calcium and mag-nesium ions are primarily derived from dissolution ofminerals in carbonate rocks such as the limestone anddolostone brought to the Otter Tail study area byglaciers.

Due to the predominance of calcium and mag-nesium ions in the water, most of the water in thestudy area is hard to very hard. Samples from 72 of74 wells and all seven of the lakes sampled exceeded150-mg/L total hardness computed from calcium andmagnesium equivalents. This is the concentration atwhich noticeable scale buildup or staining occurs(Driscoll, 1986).

Sulfate was a major anion in water samplesfrom two wells in the southwest part of the studyarea resulting in a calcium-magnesium-sulfate (Ca-Mg-SO4) water type (sites O and V, Figure 1). Thesewells are from the hummocky moraine setting and are75 feet and 86 feet deep with sulfate concentrationsof 1,000 mg/L and 400 mg/L, respectively (250 mg/Lis the U.S. Environmental Protection Agency [EPA]secondary drinking water standard for sulfate). Allother water samples had sulfate concentrations lessthan 200 mg/L. Sulfate can impart an unpleasant odorand taste and can have a laxative effect. Sulfate ismainly produced by oxidation of sulfide (mostly frompyrite) or dissolution of sulfate minerals (such as gyp-sum) in rock and soil. Detergents in householdwastes, wastewater from treatment plants, and agri-cultural fertilizers can also contribute sulfate to theshallow ground-water system (Safe Drinking WaterCommittee, 1977).

Sodium and potassium were the major cations inwater samples from three deep wells in till resulting ina sodium-potassium-bicarbonate (Na-K-HCO3) watertype (sites C, E, and J, Figure 1). Sodium concentra-tions exceeded 100 mg/L in each of these �naturallysoft� water samples; the hardness of these waterswas less than 150 mg/L. Two of these wells, 275 feetand 298 feet deep, are found in the ground morainesetting; the third is 403 feet deep and completed in aburied aquifer in till sediments beneath the outwashplain setting. Sodium and potassium ions in the sedi-ments are released from clay minerals into the groundwater when calcium and magnesium ions exchangeonto the sediments. Deeper ground water moves veryslowly and this ion exchange process has not yet de-pleted the sodium and potassium from the sediments.

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FIGURE 1. Sites with noted ground-water characteristics.The map and summary table show the relationship be-tween hydrogeologic settings and selected water charac-teristics. Labels (J, 298) identify well and depth in feet.Shading delineates hydrogeologic settings (Plate 3). SitesE and J are discussed on p.3.Water Quality

All lake-water samples and 63 of 74 ground-water samples had TDS concentrations less than 500mg/L. Ten ground-water samples from private wellsin the southwest part of the study area had TDS con-centrations greater than 500 mg/L (see Figure 1). Oneof these is a 350-foot-deep well in the collapsed out-wash setting; the other nine are in the hummocky mo-raine setting. A sample from a well in the northeast

also exceeded 500 mg/L TDS but the well is not awater supply well. High TDS (greater than 500 mg/L)can lead to well-screen incrustation. The EPA has seta secondary drinking water standard for TDS at500 mg/L.

Five ground-water samples had characteristicsthat meet criteria for nitrate vulnerability (Eh greaterthan 250 millivolts [mV], dissolved oxygen greaterthan 1 mg/L, and total iron less than 0.7 mg/L [MikeTrojan, oral commun., 1999]). If there is a localsource of nitrate, the water in these wells could be-come contaminated since denitrification would be lim-ited. In fact, three of these wells had nitrate levelsgreater than the Health Risk Limit of 10 mg/L estab-lished by the Minnesota Department of Health. Allthree are in the central part of the study area (sites G,H, and K, Figure 1).

Iron and manganese concentrations are abovethe EPA secondary standards (0.3 mg/L iron, 0.05mg/L manganese) for most of the ground-water sam-ples (65 of 74 samples exceeded the iron standardand 61 exceeded the manganese standard). High lev-els of these elements may produce objectionable odor,taste, or staining.

LAKES AND INTERACTION WITH GROUND WATER

Lakes and ground water are different expres-sions of a single resource. At many locations in thestudy area, ground water discharges to a lake andlake water recharges ground water, often at the samelake. Stable isotopes of hydrogen and oxygen andwater chemistry can be analyzed in samples ofground water and lake water to interpret theseinteractions.

Deuterium (2H) is an isotope of hydrogen con-sisting of a proton and a neutron, whereas hydrogenconsists of a proton. Deuterium, therefore, has ap-proximately twice the mass of common hydrogen.Similarly, oxygen-18 (18O) has more mass than themore common oxygen-16 (16O). Fractionation occursbecause of these mass differences. Molecules ofwater with the more common hydrogen and oxygenare lighter and more readily evaporated, leaving theremaining water more concentrated in the heavierisotopes. Because of this fractionation, lake watertypically shows an evaporative signature (a higherconcentration of the heavier isotopes than precipita-tion). Water that infiltrates the ground is not fraction-ated, so it has a meteoric signature (higherconcentration of the lighter, more prevalent isotopes).

Summary Table of Settings and Water CharacteristicsHummocky

MoraineCollapsed Outwash

Outwash Plain

High TDS (>500 mg/L)

M, N, O, Q, R, T, U, V, W

S C

NO3 – vulnerable

K G, H F, P

High Cl/Br (>450 to 1)

I, K A, G B, D, L

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FIGURE 2. Lake and well sites with stable isotope data. Thefirst number is d2H (per mil) and the second number is d18O(per mil). Shading delineates hydrogeologic settings (seePlate 3).

Analyses of stable isotopes in water samplesfrom seven lakes were compared with analyses ofstable isotopes in water samples from nearby shallowwells. Figure 2 shows the locations of the four lakes(Lida, Anna, Miltona, and Irene) in the hummockymoraine setting and three lakes (Cotton, Portage, andLittle Pine) in the outwash settings that were selectedfor comparison. Figure 3 displays a portion of theNorth American meteoric data and shows the stableisotope data from these study area sites. TDS con-centrations, electrical conductivity, and alkalinity mea-surements from these seven lakes and electricalconductivity and alkalinity measurements from anadditional 32 lakes were studied to make comparisonsand generalizations about lakes in hummocky morainesettings and lakes in the outwash settings.

Samples from two wells,one (65 feet deep) at a site onthe south side of Anna Lake inOtter Tail County and theother (70 feet deep) at a siteon the west side of CottonLake in Becker County, hadstable isotope values showingfractionation from evaporationsimilar to that seen in surfacewater. This similarity indicatesthat lake water rechargesground water at these sites(Table 1). This conclusionabout ground water and sur-face water at the Cotton Lakesite was reinforced becausethe water samples showedsimilar general chemistry re-sults when strontium, calcium,and magnesium ratios werecompared. Conversely, Por-tage Lake, which receives

FIGURE 3. Stable isotope data for the study area. Ground-water samples with anevaporative signature similar to the lake water near them are displayed in the upperright. Lake-water samples with shorter residence time or ground-water influenceplot closer to the meteoric water line.

Sample source

Well depth (feet)

Direction from lake

Water elevation

(feet)

� 2H(per mil)

� 18O(per mil)

Anna (moraine setting)Lake -- -- 1317 -44 -4.2Well 65 South 1317 -41 -4.5Well 80 West 1312 -65 -8.8

Cotton (outwash setting)Lake -- -- 1443 -46 -4.5Well 70 West 1438 -48 -4.8Well 77 East 1445 -83 -11.7

TABLE 1. Stable isotope data for selected samples from lake and ground-water sources.

[ � , relative difference from standard mean ocean water;per mil, parts per thousand; --, not applicable]

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ground water, had stable isotope values indicating lessfractionation than water samples from other lakes inthe study area because water moves quickly throughthe lake. Water samples from Lake Irene and LittlePine Lake showed some mixing of surface water andground water.

The direction of lake-water and ground-waterflow can vary throughout a lake. For example, al-though the ground-water samples from Cotton Lake�swest side and Anna Lake�s south side show strongsurface-water influences, the other ground-watersites sampled at these lakes did not (Figure 2 andTable 1). These other sites had stable isotope ratiossimilar to those of precipitation (nonfractionated) forthe study area. The direction of lake- and ground-water exchange could not be determined by stableisotope ratios in samples from lakes and ground-watersites at Lake Miltona and Lake Lida.

Most lakes in the hummocky moraine setting arein depressions without inlets or outlets. Water levels inthese lakes are controlled by evaporation and precipi-tation, and residence time is generally longer than inlakes with surface outlets such as most of those in theoutwash settings. Consequently, electrical conduc-tivity and alkalinity measurements made in the fieldwere generally higher in the moraine lakes. Ten of 14samples from moraine lakes had electrical conduc-tivity values greater than 380 microsiemens per centi-meter (mS/cm) compared with the samples fromoutwash lakes where 22 of 24 samples had valuesless than 380 mS/cm. Alkalinity values were also gen-erally higher in moraine lakes than in outwash lakes(11 of 12 samples with 180 mg/L or more and 18 of24 samples with 180 mg/L or less, respectively).Water chemistry in the moraine lakes was more vari-able than in outwash lakes. TDS ranged from 190mg/L to 300 mg/L in the moraine lakes sampled andfrom 170 mg/L to 200 mg/L in the outwash lakessampled.

GROUND-WATER RESIDENCE TIMEResidence time, indicated by isotopic and chemi-

cal data, is the time elapsed from when water leavesthe atmosphere until it is discharged from the ground-water system. Tritium and carbon-14 (14C) are isoto-pic indicators of ground-water residence time inaquifers. Seventy-four well-water samples were ana-lyzed for tritium concentrations to determine depths ofrecent water (less than 50 years old) penetration (seeFigure 1 on Plate 3). Water samples from 10 wellswere analyzed for carbon isotope concentrations.

Carbon-14 results are modeled to approximate theages of water older than 100 years (see Radioactiveand Stable Isotopes, above, for a description of iso-topes). Additionally, analyses of major ions are usedto support interpretations of residence time andrecharge source.Tritium

The coarse sediments of the outwash settingspermit relatively rapid infiltration of ground waterfrom the surface. Detectable tritium was found in allbut one of the 14 samples from wells completed atdepths of less than 135 feet in those settings.

In the outwash plain setting, all six of the sam-ples from wells less than 100 feet deep had detect-able tritium. Four of nine wells (55 feet to 82 feetdeep) sampled had recent water, and samples fromtwo of those wells contained elevated tritium concen-trations indicative of water that infiltrated during theCold War era when atomic weapons testing was at itspeak (1950�s�1960�s). The water from two wells (46feet and 74 feet deep) had an intermediate concentra-tion of tritium that indicates a mixture of recent andvintage waters. The remaining three wells sampledhad vintage water, but the wells are deeper than 135feet. Ground water within the outwash plain sedi-ments is typically years to a few decades old.

Of 11 wells sampled in the collapsed outwashsetting, six wells (less than 135 feet deep) had recentwater, including two with high tritium concentrationstypical of water recharged during the Cold War era.Two wells (70 feet and 77 feet deep) had mixedwater and three wells (64 feet, 135 feet, and 350 feetdeep) contained vintage water. The longer residencetime in the shallow well (64 feet) indicates slow, later-al ground-water movement in this area. Ground waterwithin the collapsed outwash sediments is typicallyyears to a few decades old.

The sediments in the hummocky and groundmoraine settings contain higher proportions of finesilts and clays than those of the outwash settings.These fine sediments retard the movement of groundwater, which decreases the rate of ground-waterrecharge.

In the hummocky moraine setting, half of thesampled wells (22 of 44) had no detectable tritium.The depths of these wells range from 65 feet to 384feet deep. Tritium concentrations showing mixedwater were found in 14 wells (75 feet to 239 feetdeep). Eight wells (55 feet to 142 feet deep) had re-cent water, and three of these wells had water with

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high tritium concentrations typical of water rechargedduring the Cold War era. Although half of the samplesfrom hummocky moraine were vintage water, young-er water may be found very deep where sand andgravel deposits provide pathways within the till. Withlocal exceptions, ground-water ages within the buriedaquifers in the hummocky moraine setting aredecades to centuries or older.

The ground moraine setting is characterized bylaterally extensive till that was compacted by over-riding ice. This till can greatly retard the movement ofwater resulting in long ground-water residence times.Six of 10 water samples from wells that were 135feet to 298 feet deep had no detectable tritium, andone well (179 feet deep) had a tritium concentrationthat barely put it in the mixed water category(0.9 TU). There were two more water samples withmixed water from wells 88 feet and 315 feet deep.The latter well is too deep to explain the presence ofmixed water geologically, and it is assumed this wellcould have a construction problem like that shown incondition 4 on Figure 5 of Plate 4. One well (79 feetdeep) had recent water that recharged during theCold War era. In this setting, ground-water ageswithin the buried aquifers are decades to centuries orolder.Carbon-14

Waters with short residence times in all hydro-geologic settings are readily identifiable by their tri-tium concentration. To identify residence timesexceeding 100 years, 10 wells from three hydrogeo-logic settings were sampled for carbon-14 age dating.No wells were sampled for carbon-14 in the outwashplain setting because there were few samples withvintage water and it was not expected that any ofthese waters would be more than a few hundredyears old.

Samples of vintage water were collected fromtwo wells in the collapsed outwash setting; one sam-ple had a carbon-14 age of 1,000 years and the otherof 4,000 years. Contrary to expectation, the 4,000-year-old water came from a 64-foot-deep well, andthe 1,000-year-old water from a 350-foot-deep well.These two samples show that even in the outwashsettings, there can be pockets of old water at shal-lower than expected depths. Most of the groundwater sampled in the collapsed outwash sediments,however, was less than 50 years old.

There is no clear relationship between depth andresidence time in the hummocky moraine setting.

Ground-water flow can bypass low-permeability, fine-grained sediments by traveling through interconnectedsands and gravels (see condition 3 in Figure 5 onPlate 4). To assess ground-water ages in the hum-mocky moraine setting, six water samples of vintageage were collected for carbon-14 age dating fromwells that are 183 feet to 537 feet deep. The modeledground-water ages ranged from 300 years to 8,000years.

Most of the samples were vintage water in theground moraine setting, which indicates slow re-charge to buried aquifers. One well that was sampledfor carbon-14 had a residence time of about 2,000years, even though the well is only 142 feet deep.Hydrogeochemistry and Recharge Sources

The interaction of water with different geologicmaterials, as it moves through sediments at varyingrates, can alter the chemistry of the water. Thesechemistry changes reveal information about ground-water residence times and recharge sources.

Barium and Strontium. Examples of thischemical change are shown by the concentrations ofthe naturally occurring trace elements barium (Ba)and strontium (Sr) in ground water (Figure 4). Themedian concentrations in ground-water samples withrecent water were 0.12 mg/L barium and 0.12 mg/Lstrontium, whereas ground-water samples with vin-tage water had median concentrations of 0.17 mg/Lbarium and 0.35 mg/L strontium. Clay minerals inunweathered till provide a source of barium andstrontium ions. The clay slows the movement ofground water; consequently, the time that water is incontact with these minerals is longer, allowing bariumand strontium ions in the minerals to exchange withions in the ground water through equilibrium pro-cesses. Because of the slow movement of waterthrough the fine-grained till, not enough water hasmoved through the till to deplete the barium andstrontium from the clay minerals.

In the study area, slow ground-water movementcommonly occurs in the ground moraine settingwhere the barium and strontium concentrations wereabove 0.20 mg/L in all but one of the vintage ground-water samples. The compacted sediments of theground moraine contain few sand and gravel path-ways that could reduce the contact of water with till.These clay minerals also limit the volume of waterthat passes through the till.

In contrast, the loosely deposited sediments ofthe outwash plain setting contain much less clay. This

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condition allows water to pass through these sedi-ments more quickly than through moraine and limitscontact with clay minerals. Over time, enough watermoves through these higher permeability sediments todeplete barium and strontium. All of the ground-watersamples from wells completed in the outwash plainsediments were recent or mixed water with less than0.20 mg/L barium and less than 0.10 mg/L strontium.

In both the hummocky moraine and collapsedoutwash settings, the direction and rate of ground-water flow vary because of the settings� high reliefand heterogeneities. Where these flow paths are pri-marily within sand and gravel deposits, barium andstrontium should be less than about 0.20 mg/L. How-ever, ground water from wells completed in isolatedsand bodies must pass through some clay-rich till.Under these conditions, the concentrations of bariumand strontium should be greater than about 0.20mg/L. As expected, most samples from wells in thecollapsed outwash setting had characteristics of wa-ter that moved along flow paths dominated by sandand gravel. All of those with recent or mixed waterhad concentrations of 0.20 mg/L or less barium andless than 0.20 mg/L strontium. In the hummocky mo-raine setting, few wells are completed in sediments

with uninterrupted flow paths of sand and gravel. Inthis setting, 27 of 44 samples exceeded 0.20 mg/Lstrontium (20 vintage water samples and sevenmixed water samples). All samples of the recentground water from the hummocky moraine settinghad strontium concentrations of less than 0.20 mg/L.Barium concentrations had the broadest range inhummocky moraine, from 0.01 mg/L to 1.38 mg/L.

Two noteworthy trends are shown on Figure 4.Recent water generally has lower barium and stron-tium concentrations (less than 0.20 mg/L), and vin-tage water has decreasing barium concentrationswith increasing strontium concentrations. This in-verse relationship between barium and strontium isshown in about half of the water samples from wellsin the hummocky moraine (Figure 4, vintage water onthe right side of plot). A possible explanation is thatthese waters recharged through wetlands, whereorganic-rich sediments produce hydrogen sulfide gas(H2S) (see Figure 5). The hydrogen sulfide gas mi-grates upward creating zones of high sulfate wherebarium is removed from the ground water by precipi-tating as the mineral barite (BaSO4).Figure 4 also shows that many samples of ColdWar era ground water have concentrations of barium

and strontium similar to lake water. Wa-ter with an elevated tritium concentra-tion (Cold War era) may have enteredthe ground water from a lake, ratherthan from infiltration by precipitationfrom the Cold War era. A lake rechargesource for these wells could not be veri-fied because no lake-water sampleswere analyzed for tritium as a part ofthis study.

Chloride. The presence of chlo-ride (Cl) from human activities may indi-cate relatively recent recharge ofground water. Chloride is a componentof road salt, agricultural fertilizer, sep-tage, and water softening salt. Waterthat contains chloride derived from hu-man activities is generally very depletedin bromide (Br) compared with chloride.For example, sodium chloride salts de-rived from halite deposits (commonlyused in industrial, agricultural, and do-mestic applications) typically have chlo-ride to bromide ratios greater than10,000 to 1. Seven of the samples withrecent water had chloride to bromideratios greater than 1,000 to 1, indicating

FIGURE 4. Comparison of the cations barium and strontium bytritium age and hydrogeologic setting. Most of the recent water samplesshown are from wells in outwash with lower concentrations of barium andstrontium.These cations are released into ground water from tills wherewater movement is slower and the volume of water moving through thesesediments is lower. Consequently, the highest concentrations of bariumand strontium occur in the vintage water samples (mostly from the morainesettings). The trend for some of these vintage samples to have decreasingbarium with increasing strontium is explained in the text.

a human impact (Figure 1). In contrast, rainwaterwith natural sources of chloride has chloride to bro-mide ratios near 300 to 1. The two samples with thehighest chloride concentrations are both vintage watersamples with chloride to bromide ratios of 400 to 1,indicating a natural source of chloride.

A second geochemical indicator of relativelyrecent recharge of ground water is evidence of am-monia fertilizers in ground water. Anhydrous ammo-nia, a commonly applied fertilizer, is a very strongcation exchanger. Both anhydrous ammo-nia and ammonia from animal manuredisplace cations held naturally on clayminerals in the soil. Calcium and magne-sium are the dominant cations on the clayminerals but are held less strongly thanstrontium. Fertilizer also provides asource of chloride ions, which do not in-teract with the clay minerals. Therefore,comparing the chloride concentration tothe molar ratio of strontium to calciumplus magnesium (Figure 6) can help sepa-rate natural processes from human activi-ties. Recent ground water, as expected,shows proportionally low strontium tocalcium plus magnesium ratios and oftencontains elevated chlorides. In contrast,vintage water contains a higher propor-tion of strontium to calcium plus magne-sium.

Recharge Sources. Identifyingthe source of older waters is importantfor protecting future ground-watersupplies. Geochemical evidence canseparate these older waters into aqui-fers that received either wetland orupland recharge. Minnesota rainwatertypically contains about 0.5 mg/L to 2mg/L chloride, and in wetlands, theevaporative concentration of chloridefrom rainwater is relatively small.Consequently, ground water rechargedfrom wetlands will typically containless than 3 mg/L chloride. Fifty-one of75 ground-water samples from wellsthroughout the study area had a wet-land recharge signature and 25 ofthese had vintage water (Figure 6).The depths of these wells ranged from46 feet to 537 feet deep. In uplandsoils, on the other hand, evapotranspi-

ration can remove most of the available water, pro-ducing chloride concentrations of up to 20 mg/L.Eight ground-water samples from wells ranging indepth from 65 feet to 403 feet deep had an uplandrecharge signature, and all but one of these hadvintage water.

Wetlands represent the largest sources ofground-water recharge for this study area (Figure 6).Until the settlement of Minnesota in the 1860�s, about

FIGURE 5. Chloride and sulfate in Minnesota waters showing origin ofelevated chloride from uplands and low chloride from wetlands.

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FIGURE 6. Recharge environment based on comparison of chlorideconcentration to the molar ratio of strontium (Sr) to calcium (Ca) plusmagnesium (Mg).

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18.6 million of its 53.6 million acres were wetlands.Today only half of that wetland acreage remains(Minnesota Board of Water and Soil Resources,[n.d.]). Continued preservation of wetlands will helpensure recharge for future water supplies.

Attention can be focused on areas most suscep-tible to contamination from human activities, such asin recharge areas and geologic settings that producedeeper ground-water flow. In such areas, ground-water pumping for municipalities, industry, and agri-culture can reduce residence times and draw moderncontaminants deeper more quickly than they wouldreach under undisturbed conditions. Additionally,efforts to look for specific contaminants, like agricul-tural pesticides or human pathogens, can focus onground waters with rapid recharge rates in areas ofintensive land use.

SENSITIVITY MAP PREPARATIONGeologic sensitivity to pollution can be evaluated

by relating it to the travel time of water to a target ofinterest. Shorter travel times may indicate higher sen-sitivity to pollution. Longer travel times may representan increased level of geologic protection for an aqui-fer. The map on Plate 4 is an interpretation of thegeologic sensitivity to pollution of a defined targetzone in the Otter Tail study area. These sensitivityand travel-time relationships are shown in Figure 1 onPlate 4. The method for determining this geologicsensitivity is described below.

Four main hydrogeologic and geologic factorswere identified that influence vertical travel time ofwater: hydrogeologic setting, surficial sediment,unsaturated thickness, and target zone lithology. Twofactors are based on spatial data from Plate 3 andfrom Plate 1 of Part A (hydrogeologic setting andsurficial sediment, respectively), and two are basedon point data from field measurements and well logs(unsaturated thickness and target zone lithology).Each factor was subdivided into categories. Thefactors and their associated categories (in parenthe-ses) are hydrogeologic setting (outwash plain, col-lapsed outwash, hummocky moraine, groundmoraine), surficial sediment (sand and gravel, sandyloam, loam, loam to clay loam), unsaturated thickness(0 to 20 feet, 21 to 50 feet, greater than 50 feet), andtarget zone lithology (no clay, some clay, all clay). Avalue representing the vertical time of travel forwater to reach a depth of 20 to 50 feet (the targetzone) was assigned to each of the categories.

The study area was divided into 83 map poly-

gons to apply the factors to an assessment of geologicsensitivity. The boundaries of a polygon enclose ageographic area with a single hydrogeologic settingand surficial sediment, and an overall similarity inunsaturated thickness and target zone lithology.Spatial Data

Vertical ground-water travel time to the targetzone was approximated for each category within aspatial factor based on hydraulic conductivity valuesfrom literature (Stephenson and others, 1988, and SoilSurvey staff, 1998). These hydraulic conductivityvalues represent vertical flow rates through saturatedporous sediment. Some or all of the sediment abovethe target zone is unsaturated throughout most of thisstudy area. Except for flow through fractures andother pathways, ground-water flow through unsatur-ated sediments is slower than flow through saturatedsediments. Applying the faster saturated flow rates tounsaturated sediment may result in a shorter thanrealistic travel time. However, this effect is dimin-ished by the unsaturated thickness category, whichassigns longer travel times to thicker unsaturatedsediment.

Each category within the setting and sedimentfactors was given a value on a scale of 1 to 5 corre-sponding to the range of travel time (Figure 1 on Plate4) for water to enter the target zone: 1 = hours tomonths, 2 = weeks to years, 3 = years to decades,4 = decades to a few centuries, and 5 = decades toseveral centuries. The categories within the settingsfactor and their associated travel time value inparentheses are outwash plain (1), collapsed outwash(2), hummocky moraine (3), and ground moraine (4).The categories within the sediment factor and theirassociated travel time values are sand and gravel(1.5), sandy loam (2.5), loam (4), and loam to clayloam (4.5).Point Data

The point data were assigned a numerical valuefor each category. The unsaturated thickness factorwas divided into the following categories: 0 to 20 feet(some to all of the sediment above the target zone isunsaturated) = 1, 21 feet to 50 feet (the unsaturatedsediment continues into the target zone) = 2, andgreater than 50 feet (the sediment is unsaturated tobelow the target zone) = 3. A representative value foreach map polygon was calculated based on thepercentage of data points within each of these

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categories. This representative value was adjusted toa scale of 1 to 5 to correspond to the spatial datascale. The target zone lithology factor accounts forthe possibility that sediments within the target zoneare different from those mapped at the surface, adifference that would affect vertical time of travel.This factor was divided into three categories: noclay = 1, some clay = 2, and all clay = 3. Again, arepresentative value for each map polygon wascalculated and adjusted to a scale of 1 to 5.Sensitivity Rating

The sum of the numerical values for the fourfactors was averaged for each map polygon, and aninitial sensitivity rating was assigned based on thataverage as follows: 1 to 1.7 = very high, 1.8 to 2.5 =high, 2.6 to 3.4 = moderate, 3.5 to 4.2 = low, and 4.3to 5 = very low. The matrix in Figure 3 on Plate 4shows the results of this process. Sensitivity ratings ofmap polygons that lacked point data were comparedwith similar neighboring map polygons to determinetheir geologic sensitivity rating. If the average valuefor a map polygon fell close to a boundary betweentwo ratings, the value for that map polygon was re-examined using qualifying information.

Qualifying information was used to validate oradjust the initial sensitivity rating for each mappolygon based on the physical and chemical environ-ments. The following are examples of features of thephysical environment. Many isolated lakes or wet-lands in low-lying areas of a map polygon suggestsluggish ground-water movement. Well-developedstream networks (dendritic streams) suggest apreference for precipitation to run off rather thaninfiltrate. Either of these conditions could result in alower sensitivity rating. The chemical environmentwas also examined to validate or adjust sensitivityratings. The presence of tritium in ground-watersamples or evidence of enhanced recharge, forexample, ruled out the possibility of a low sensitivityrating.Examples of the Process

The following two examples illustrate theprocess described above. The factors and time oftravel values (in parentheses) for a map polygon inthe area of Sebeka were the following: groundmoraine (4), sandy loam (2.5), unsaturated thickness(averaged 2.0 on a scale of 1 to 3; adjusted to 3 on ascale of 1 to 5), and target zone lithology (averaged

2.5 on a scale of 1 to 3; adjusted to 4 on a scale of1 to 5). These values were summed: 4 + 2.5 + 3 + 4 =13.5 and then averaged: 13.5 ¸ 4 = 3.4. This valuecorresponded to an initial sensitivity rating of moder-ate but was close to the value for a low rating.Qualifying information was considered to validate oradjust the rating of the map polygon. This is an areaof well-developed stream drainage, which suggeststhat water moves out of this area rather than infiltrat-ing deeply. Additionally, there was no detectabletritium from water samples in this map polygon;therefore, the final rating for this map polygon wasadjusted to low.

Another example is a map polygon that runsfrom Wadena to the east. Its categories with time oftravel values in parentheses were the following:outwash plain (1), sand and gravel (1.5), unsaturatedthickness (averaged 2.2 on a scale of 1 to 3; adjustedto 3 on a scale of 1 to 5), and target zone lithology(averaged 1.4 on a scale of 1 to 3; adjusted to 2 on ascale of 1 to 5). These values were summed: 1 +1.5 + 3 + 2 = 7.5 and then averaged: 7.5 ¸ 4 = 1.9,yielding an initial sensitivity rating of high. The finalsensitivity rating was adjusted to very high becausefield-measured data (depth to water) showed theunsaturated thickness was thinner than when histori-cal data (from the County Well Index) were includedin the analysis. Additionally, the flat terrain in coarsesediments means that precipitation can infiltratereadily.

REFERENCES CITEDAlexander, S. C., and Alexander, E. C., Jr., 1989,

Residence times of Minnesota groundwaters:Minnesota Academy of Sciences Journal, v. 55,no. 1, p. 48-52.

Driscoll, F. G., principal author and ed., 1986, Ground-water and wells (2d ed.): St. Paul, Minn.,Johnson Division, p. 91.

Hem, John D., 1992, Study and interpretation of thechemical characteristics of natural water (3ded.): U.S. Geological Survey Water-SupplyPaper 2254, 263 p., 3 pls. in pocket.

Minnesota Board of Water and Soil Resources, [n.d.],Wetlands in Minnesota brochure: St. Paul,Minnesota Board of Water and Soil Resources.

Safe Drinking Water Committee, 1977, Drinkingwater and health: Washington, D.C., NationalAcademy of Sciences, p. 425, 939.

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Soil Survey staff, 1998, Appendix, in Keys to soiltaxonomy (8th ed.): U.S. Department of Agricul-ture, Natural Resources Conservation Service,p. 317.

Stephenson, D. A., Fleming, A. H., and Mickelson, D.M., 1988, Glacial deposits in Back, W.,Rosenshein, J. S., and Seaber, P. R., eds.,Hydrogeology: Boulder, Colo., GeologicalSociety of America, The Geology of NorthAmerica, v. O-2, p. 304-312.